From History.com:
Memorial Day is an American holiday, observed on the last Monday of May, honoring the men and women who died while serving in the U.S. military. Memorial Day 2022 will occur on Monday, May 30.
Originally known as Decoration Day, it originated in the years following the Civil War and became an official federal holiday in 1971. Many Americans observe Memorial Day by visiting cemeteries or memorials, holding family gatherings and participating in parades. Unofficially, it marks the beginning of the summer season.
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The Civil War, which ended in the spring of 1865, claimed more lives than any conflict in U.S. history and required the establishment of the country’s first national cemeteries.
By the late 1860s, Americans in various towns and cities had begun holding springtime tributes to these countless fallen soldiers, decorating their graves with flowers and reciting prayers.
It is unclear where exactly this tradition originated; numerous different communities may have independently initiated the memorial gatherings. And some records show that one of the earliest Memorial Day commemorations was organized by a group of formerly enslaved people in Charleston, South Carolina less than a month after the Confederacy surrendered in 1865.
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Decoration Day
On May 5, 1868, General John A. Logan, leader of an organization for Northern Civil War veterans, called for a nationwide day of remembrance later that month. “The 30th of May, 1868, is designated for the purpose of strewing with flowers, or otherwise decorating the graves of comrades who died in defense of their country during the late rebellion, and whose bodies now lie in almost every city, village and hamlet churchyard in the land,” he proclaimed.
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On the first Decoration Day, General James Garfield made a speech at Arlington National Cemetery, and 5,000 participants decorated the graves of the 20,000 Civil War soldiers buried there.
Many Northern states held similar commemorative events and reprised the tradition in subsequent years; by 1890 each one had made Decoration Day an official state holiday. Southern states, on the other hand, continued to honor the dead on separate days until after World War I.
Link to the rest at History.com